This present invention relates to the field of signal decoding, and in particular to decoding of biphase signals by which digital information is transmitted.
The bits of the original digital information are encoded prior to transmission, and then transmitted as “symbols”. A transmission technique using biphase signals is Binary Phase-Shift Keying (BPSK). To reduce the bandwidth requirement for the transmission of these digital signals, Nyquist filters are used at the transmitter and receiver ends for pulse regeneration and signal recovery. The steep pulse edges are replaced by slowly rising transition curves provided with overshoot, whereby undesired harmonics are largely suppressed. In the limiting case, rather than a pulse signal, a bell-shaped signal with overshoot whose zero crossings are defined by constant symbol intervals T is transmitted. The frequency requirement in such a transmission is about twice as high as the data rate to be transmitted. Nevertheless, a reliable distinction between “1” and “0” states is ensured at the sampling instants, because the individual symbols are shifted in time with respect to each other by integral multiples of T and because the zero crossings all coincide.
European Patent Application EP-A 0 912 010 discloses a typical receiver for digitally transmitted signals that incorporates such Nyquist filters. To distinguish between the logic states of the individual symbols, a sampling control loop is provided that optimizes the sampling instant for the individual symbols (timing recovery).
A property of digital signals may be that the digital states of the signal to be transmitted are not uniformly distributed. At the transmitter and receiver ends, this may have an effect like a superimposed dc level, which is undesirable in many cases. This can be remedied using suitable coding techniques. Each bit to be transmitted is transmitted by a balanced bit combination, with the original state value being encoded via the sequence of associated bits. Such coding techniques can also be used in an interleaved scheme, or be combined with other coding methods.
Biphase coding is used, for example, to transmit additional digital information in a radio broadcast signal. This additional information is known as a Radio Data System (RDS) signal. In many countries RDS transmits specific information to motorists within predetermined FM stereo broadcast channels. With the digital RDS signal, a 57-kHz subcarrier locked to a 19-kHz pilot signal is modulated by a double modulation. The biphase coding replaces a logic “0” by the combination “0, 1”, and a logic “1” by the combination “1, 0”. Each logic state of the original data sequence thus generates two successive bits, which are referred to as a bit combination or symbol pair. This type of biphase coding ensures that a symbol change occurs in each symbol pair, so that the data clock can be relatively easily recovered from the received data sequence.
At the receiver, the 57-kHz biphase signal is synchronously downconverted to baseband and band-limited by digital signal processing. As the biphase signal is basically a phase modulation of the 57-kHz carrier, it is also possible to use a complex vector analysis. Theoretically, this provides a rotating vector that comes to rest after the mixing at 57 kHz. A carrier/phase recovery circuit rotates the complex vector to the real axis, so that imaginary components are no longer present. A Cordic algorithm may then be used, for example, to determine phase and magnitude.
At the sampling instants on the symbol rate, the phase is ideally determined by two states, whereas the magnitude should be constant. In reality, these theoretical states are only approximated, and they are additionally falsified by interference signals. From the phase values obtained, the two logic states “0” or “1” are decided and a corresponding bit sequence is formed. From this bit sequence, two-bit groups are formed that must have a bit change in accordance with the coding at the transmitter end. The formation of the two-bit loops may be based on recognition that during the formation of the groups, the number of unallowable symbol pairs in the form of “0, 0” and “1, 1” groups becomes a minimum. In a last step, each two-bit group is assigned either a logic “0” or a logic “1”, whereby the original data sequence is restored at the receiver. However, due to transmission errors, there are erroneous groups with “0, 0” and “1, 1” states.
Therefore, there is a need for a system and method of detecting and correcting transmission errors in biphase signals, so that satisfactory data transmission is ensured even in the presence of heavy interference.